An undetermined volume of sewage leaked from a pipe at the state's largest wastewater treatment plant early Saturday and flowed into the Kinnickinnic River at the inner harbor, Milwaukee Metropolitan Sewerage District officials said Monday.

Workers observed a pool of untreated sewage on the surface near the west end of Jones St. at the Jones Island treatment plant around 4:30 a.m. Saturday, MMSD contract compliance officer Patrick Obenauf said.

The wastewater was seen draining into a nearby storm sewer that discharges to the river and workers immediately reduced pressure in several underground sewer pipes at that location to stop the leak, Obenauf said.

No sewage was reported on the surface there at the time of the previous visual inspection of the plant more than 10 hours earlier at 6 p.m. Friday, he said.

MMSD Executive Director Kevin Shafer on Monday used his emergency authority to hire an excavation contractor to remove soil there this week and determine the location of the leak. Cost of the excavation is estimated at $70,000.

The site is adjacent to a shaft containing multiple pipes used to lift sewage up to the plant from harbor siphons. The $138 million siphon project was designed to reduce sewer overflows to local rivers and Lake Michigan.

The two siphons constructed from 2007 to 2009 — one each beneath the Milwaukee and Kinnickinnic rivers — increased the volume of sewage that could be pumped from regional sewers directly to Jones Island during heavy rainstorms.

The new siphons prevent millions of gallons of wastewater a day during storms from going into the deep tunnel and filling the storage system too quickly. Widespread overflows can occur when the tunnel is closed during intense storms.

But the siphons must operate under full pressure during storms or too much sewage would be diverted to the deep tunnel, Obenauf said. For that reason, repairs must be done as soon as possible to ensure the siphons are operating at capacity before the spring wet weather season.